Adding verbiage to the Language Extensions document about __has_include and...

Adding verbiage to the Language Extensions document about __has_include and __has_include_next only being allowed within preprocessor directives.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172643 91177308-0d34-0410-b5e6-96231b3b80d8

docs/LanguageExtensions.rst

@@ -140,7 +140,8 @@ Include File Checking Macros
 Not all developments systems have the same include files.  The
 :ref:`langext-__has_include` and :ref:`langext-__has_include_next` macros allow
 you to check for the existence of an include file before doing a possibly
-failing ``#include`` directive.
+failing ``#include`` directive.  Include file checking macros must be used 
+as expressions in #if or #elif preprocessing directives.
 
 .. _langext-__has_include: